This invention relates to planar optical waveguides which contain specific components, or planar optical elements.
A planar optical element, as used in this application, is defined as any integrated optical element formed in the optical path of a planar optical waveguide, including lenses, gratings, and microprisms. The preceding list is illustrative only and is not intended to be all-inclusive.
Several techniques are known for producing planar optical elements in planar integrated optical devices. These methods include: geodesic components (see, e.g., Nicia U.S. Pat. No. 4,712,856); Fresnel lenses (see, e.g., Suhara et al., "Graded-Index Fresnel Lenses for Integrated Optics," Applied Optics, vol. 21, no. 11, pp. 1966-71, Jun. 1, 1982; Luneberg lenses (see, e.g., Columbini, "Design of Thin-film Luneberg-type Lenses for Maximum Focal Length Control", Applied Optics, vol. 20, no. 20, pp. 3589-93, Oct. 5, 1981; and grating lenses (see, e.g., Hatakoshi et al., "Waveguide Grating Lenses for Optical Couplers", Applied Optics, vol. 23, no. 11, pp. 1749-53, Jun. 1, 1984). Another technique has been developed wherein planar optical waveguides and components therein are fabricated using polymers, e.g., Fan et al., EPO Patent Publication No. 0 446 672. Yet another technique involves the use of a low index lens material embedded in a high index planar waveguide, e.g., Minot et al., "A New Guided-Wave Lens Structure", Journal of Lightwave Technology, vol. 8, no. 12, pages 1856-65, December, 1990.
Geodesic lenses are characterized by a surface indentation in the top of the planar optical waveguide. One problem with the geodesic lens is the requirement of tight control during the manufacture of this surface indentation. This tight control during fabrication is required to keep scattering losses at transition points to a minimum. Also, geodesic lenses may not be suitable if additional layers of material need to be deposited over such lenses.
Luneberg lenses, which are a subclass of geodesic lenses, require the use of a lens material which has a higher index of refraction than the planar optical waveguide substrate with which it is used. This may be difficult depending on the refractive index of the planar optical waveguide substrate material, especially if the refractive index of the planar optical waveguide substrate is relatively high. Also, Luneberg lenses are extremely sensitive to small variations in deposition thickness profile.
Fresnel lenses, which are similar to zone plates in bulk optics, rely on phase shifting and/or absorption to obtain the desired focusing effect. This phase shifting is achieved through a series of half-period zones which are applied to a planar optical waveguide. Fresnel lenses exhibit unacceptable wavelength sensitivity, and overclad problems have not been solved. Also, Fresnel lenses, as well as grating lenses, exhibit poor off-axis performance and high chromatic aberration. Fabrication of the half-period zones of a Fresnel lens is difficult to control. For a more detailed discussion of the use of Fresnel lenses in planar optical waveguides, see Ashley et al., "Fresnel Lens in a Thin-film Waveguide", Applied Physics Letters, vol. 33, pages 490-92, Sept. 15, 1978.
Fan et al. EPO Patent Publication No. 0 446 672 is directed to the manufacture of planar optical waveguides from an epoxy polymer material. The planar optical waveguides in Fan et al. exhibited a sharp increase in optical loss above about 230.degree. C., the point at which the polymer began to decompose. The only planar structures investigated in Fan et al. were intersections of planar waveguides and "sharp-corner" waveguide bends. The polymer-based waveguides in Fan et al. were not overclad at all. This resulted in air as the cladding throughout the planar waveguide. This would produce a very high index difference structure which would not be compatible with singlemode and multimode fibers of the type used in telecommunications.
Minot et al. is directed to the use of a lens material with a refractive index which was lower than the refractive index of the planar waveguide. The planar waveguide was fabricated from a high-index III-V compound. By using Corning 7059 glass as the lens waveguiding component, Minot et al. was able to achieve relatively large index differences between the lens region and the host waveguide. This produced lenses with better off-axis performance, lower chromatic aberration, and higher polarization independence than were previously possible. However, Minot et al. is only applicable to III-V compound (e.g., GaAs) planar waveguides and would not produce similar results in planar waveguides based on SiO.sub.2 or doped-SiO.sub.2 materials such as those widely used in telecommunications applications because the refractive index differential between Corning 7059 glass and silica or doped-silica materials is substantially less than the refractive index differential between Corning 7059 glass and a high-index III-V compound. Also, Minot et al. does not disclose the presence of an overclad layer. This would produce a very high index difference structure over the entire optical path of the planar optical waveguide and would not be compatible with singlemode and multimode fibers of the type used in telecommunications due to high optical losses and mode-field diameter mismatch. Additionally, the absence of any overclad layer would allow changes in the medium adjacent the planar optical waveguide to affect propagation of light in the core layer and would expose the core layer to possible mechanical damage.
Other techniques for producing planar optical elements in planar optical waveguides have been developed.
Spillman et al. U.S. Pat. No. 4,547,262 discloses a method for manufacturing a planar optical waveguide on a substrate of LiTaO.sub.3. The process provides for selectively modifying the refractive index of the substrate material through ion exchange techniques to provide predetermined optical geometries. However, Spillman et al. does not disclose the use of any overclad layer. Therefore, the planar optical waveguide produced by this method would have a very high index difference structure over the entire optical path and would not be compatible with singlemode and multimode fibers of the type used in telecommunications.
Aagard et al. U.S. Pat. No. 4,141,621 discloses a method for manufacturing planar optical waveguides made of Nb.sub.2 O.sub.5. A layer of TiO.sub.2 is placed between two layers of Nb.sub.2 O.sub.5. The TiO.sub.2 layer serves the function of an etch stop to provide for more accurate etching of the top Nb.sub.2 O.sub.5 layer. By etching the top layer of N.sub.b 2O.sub.5, the effective refractive index of the area etched is modified. The area etched is in the shape of a lens. Aagard et al. discloses a problem with this method--the effect of the addition of the TiO.sub.2 layer: "Introduction of the TiO.sub.2 layer for stop-etch purposes causes an increase in waveguide loss and also affects some of the other propagation characteristics. Therefore, the TiO.sub.2 must be kept as thin as possible and still provide effective stop-etch characteristics." Aagard et al., col. 3, lines 33-38.
Stoll et al. U.S. Pat. No. 4,755,014 discloses a planar optical waveguide structure in which two surface layers of different effective refractive index are placed contiguously on a substrate. A refractive interface will thereby be provided between the two surface layers. This refractive interface can function as a lens, prism, or planar-to-channel optical waveguide interface, depending on the shape, dimensions and refractive index differential. However, Stoll et al. does not disclose an overclad layer. Therefore, the planar optical waveguide produced by this method would have a very high index difference structure over the entire optical path and would not be compatible with singlemode and multimode fibers of the type used in telecommunications.
Yet another method involves planar optical elements formed in planar optical waveguides consisting of Si.sub.3 N.sub.4 guiding layers with SiO.sub.2 cladding layers. This method is described in the following series of articles and patents: Mottier et al., "Integrated Fresnel Lens on Thermally Oxidized Silicon Substrate", Applied Optics, vol. 20, no. 9, pages 1630-1634, May 1, 1981; Valette et a)., "Integrated-optical Circuits Achieved by Planar Technology on Silicon Substrates: Application to the Optical Spectrum Analyser", IEE Proceedings, vol. 131, pt. H, no. 5, pages 325-31, October, 1984; Lizet et al. U.S. Pat. No. 4,740,951; Gidon et al. U.S. Pat. No. 4,786,133; and Gidon et al. U.S. Pat. No. 4,865,453.
Mottier et al. discloses a planar optical waveguide with Fresnel lenses. The Fresnel lens is chemically etched into the SiO.sub.2 "overlayer" to produce a change in the effective refractive index of the planar waveguide in the area of the lens. There is no cavity disclosed or suggested by Mottier et al. nor is there any suggestion or disclosure of an overclad layer on top of the etched lens.
Valette et al. discloses a variety of optical components designed into silicon-based planar optical waveguides. The planar optical waveguide structure consists of: (1) a silicon substrate, (2) a 1-4 .mu.m thick layer of silica obtained by thermal oxidation of the silicon substrate, (3) a thin layer of silicon nitride, and (4) a silica overlayer. The refractive index differential is obtained by local etching of the SiO.sub.2 overlayer. Valette et al., pages 327, 328. Valette et al. does not disclose any overcladding applied over the etched optical components nor does it disclose a cavity as part of the optical components.
Lizet et al. discloses a planar optical waveguide which can function as a multiplexer or a demultiplexer. The basic structure of the waveguide is similar to that described in Valette et al. The are several optical components which work in combination to perform the desired function of the waveguide. Diffraction gratings serve to separate the input light beam into discrete beams carrying a given wavelength. These gratings are obtained by etching either only the top silica layer of the waveguide or by etching the top silica layer and partially etching the silicon nitride layer. The gratings may be overclad, but if an overclad layer is applied, no cavities are left in the area of the gratings. There are also several mirrors which serve to reflectively focus the light onto the gratings or onto output microguides. These mirrors are obtained by etching through the top three layers of the waveguide structure. Lizet et al. does not disclose or suggest overcladding the mirror components.
Gidon et al. '133 discloses a planar optical waveguide which functions as a multiplexer or demultiplexer. The basic structure of the planar optical waveguide is similar to that described in Valette et al. above. A principal optical component of the device is a diffraction grating. This grating is obtained by etching through the top three layers of the waveguide structure. The grating has facets whose two foci coincide respectively with the input and output portions of the planar optical waveguide. The functional surface of the grating can be coated with a metal layer to enhance its reflective characteristics. Gidon et al. '133 does not disclose or suggest overcladding the grating.
Gidon et al. '453 discloses a planar optical waveguide which functions as a displacement transducer. The basic structure of the planar waveguide is similar to that described in Valette et al. above. Some of the optical components are obtained by local etching of the top layer of silica as previously described in Valette et al. Two mirror components are obtained by local etching of the three top layers of the waveguide. Gidon et al. 453 does not disclose or suggest overcladding of the mirror components.
It is an object of this invention to provide a planar optical waveguide, compatible with the needs of telecommunications applications, with at least one planar optical element embedded in the planar optical waveguide, the planar optical element comprising at least one optically functional interface adjacent to a cavity, wherein there is a substantial difference between the refractive indices of the cavity and the adjacent core regions of the planar optical waveguide.
It is another object of this invention to provide a planar optical waveguide with at least one planar optical element embedded in the planar optical waveguide, the planar optical element comprising at least one optically functional interface adjacent to a cavity, wherein the planar optical waveguide provides the function of an M.times.N coupler.
It is another object of this invention to provide a planar optical waveguide with at least one planar optical element embedded in the planar optical waveguide, the planar optical element comprising at least one interface adjacent to a cavity, wherein the planar optical element is mechanically and optically protected by an overclad layer.
It is another object of this invention to provide a method for manufacturing planar optical waveguides which are the subject of this invention.